1. Field of the Invention
The present invention relates generally to data storage systems having optical data tracking, storage and retrieval systems. More particularly, the present invention relates to data storage, tracking, and retrieval systems that include optics.
2. Background Art
In data recording and retrieval systems, using a moving media having a varying material characteristic, detectable variations from previously encoded media locations may be retrieved using incident light reflected from the media. Such variations may also be used in providing servo control signals for following previously recorded data tracks.
In a magneto-optical storage system, using a magneto-optical (MO) recording material deposited on a rotating disk, information may be recorded on the disk as spatial variations of magnetic domains. During readout, the magnetic domain pattern modulates an optical polarization, and a detection system converts a resulting signal from optical to electronic format.
In one type of magneto-optical storage system, a magneto-optical head assembly is located on a linear actuator that moves the head linearly along a radial direction of the disk to position the head assembly over data tracks during recording and readout. A magnetic coil creates a magnetic field that has a magnetic component in a direction perpendicular to the disk surface. A vertical magnetization vector in the MO medium with polarity opposite to that of the surrounding magnetic material of the disk medium is recorded as a mark indicating zero or a one by first focusing a beam of laser light to form an optical spot on the disk. The optical spot functions to heat the magneto-optical material to a Curie point (i.e., a temperature at which the magnetization may be readily altered with an applied magnetic field). A current passed through the magnetic coil orients the spontaneous vertical magnetization vector into or out of the disk surface. This orientation process occurs in the region of the optical spot where the temperature is suitably high. The orientation of the magnetization mark is preserved after the laser beam is removed. The mark is erased or overwritten if it is locally reheated to the Curie point by the laser beam while the magnetic coil creates a magnetic field in the opposite direction.
Information is read back from a particular mark on the disk by taking advantage of the magnetic Kerr effect to detect a Kerr rotation of the optical polarization that is imposed on a reflected beam by the magnetization at the mark of interest, the magnitude of the Kerr rotation being determined by the material's properties (embodied in the Kerr coefficient). The sense of the rotation is measured by established differential detection schemes as being clockwise or counter-clockwise depending on the direction of the spontaneous magnetization at the mark of interest.
Winchester magnetic hard disk technology has historically been limited by at least two factors, including: magnetic head design and the magnetic storage media. A typical prior art magnetic storage system includes a magnetic head having a slider element and a magnetic read/write element and is coupled to a rotary actuator magnet and coil assembly by a suspension and rotary actuator arm so as to be positioned over a surface of a spinning magnetic disk. In operation, lift forces are generated by aerodynamic interactions between the magnetic head and the spinning magnetic disk. The lift forces are opposed by equal and opposite spring forces applied by the suspension such that a predetermined flying height is maintained over a full radial stroke of the rotary actuator arm above the surface of the spinning magnetic disk.
Flying head designs have been proposed for use with magneto-optical (MO) storage technology. One motivation for using magneto-optical technology stems from the availability of higher areal density magneto-optical storage disks than magnetic storage disks. However, despite the historically higher areal storage density available for use with magneto-optical disks drives, the prior art MO disk drive volumetric storage capacity has generally not kept pace with the volumetric storage capacity of magnetic disk drives.
One factor limiting MO disk drives has been the physical size of the head necessary to hold the various components required for accessing magneto-optical information. Various MO flying head designs incorporating MO technology are described in U.S. Pat. No. 5,295,122 by Murakami, including: use of free-space alignment of a laser beam with a dynamically moving target, and a number of different configurations of the magnetic and optical elements required for detection of the magneto-optical Kerr effect. In Murakami, the large size and mass of the optical elements limits the minimum head size and, therefore: the speed at which information from the MO disk may be accessed, the tracking bandwidth, and the track density that may be read or written. In the prior art, the large physical size of MO flying heads also limits the spacing between magneto-optical disks to a finite minimum value and, therefore, limits the volumetric storage capacity which may be achieved.
A method for moving a folding prism or mirror with a galvanometer actuator for fine tracking has been disclosed by C. Wang in U.S. Pat. No. 5,243,241. The galvanometer consists of bulky wire coils and a rotatable magnet mounted on a linear actuator arm attached to a flying magneto-optical head, but not mounted on the slider body itself. This design limits the tracking servo bandwidth and achievable track density due to its size and weight. Its complexity also increases the cost and difficulty of manufacture.
Miniature torsional scanning mirrors have been described, viz, "Silicon Torsional Scanning Mirror" by K. Petersen, IBM J. Res. Develop., Vol. 24, No. 5 September 1980, pp. 631-637. These mirrors are generally prepared using procedures developed in the semiconductor processing arts. Petersen describes a torsion mirror structure having a 134 .mu.m thick silicon wafer defining a distal frame suspending a central silicon mirror element suspended by lateral torsion members therebetween. The lateral mirror dimensions are about 2.1 by 2.2 mm. The mirror is bonded over a 7 to 10 .mu.m deep etched well in a glass slide substrate, having evaporated electrodes deposited therein. The mirror is rotationally deflected by voltages applied between the mirror and the electrodes by connecting wires. Scanning angles of up to 2 degrees at a resonant operating frequency of up to 15 kHz were reported. The size and mass of the mirror limited higher operating frequency. Also, mirror distortion caused by the high dynamic torque (i.e. peak angular acceleration ) at higher frequency was a limiting factor.
An improved magneto-optical storage system is described in commonly assigned U.S. patent application titled "Maximum Data Storage For Magneto Optical System", Ser. No. 08/844,208 filed Apr. 18, 1997. The aforementioned application describes an RF modulated Fabry-Perot (FP) laser source that is coupled to a single optical fiber which directs the incident light to a servo controlled mirror for direction toward the disk. The mirror also directs a reflection of the incident light which carries rotated tracking and phase information from the disk. This system provides lower cost, mechanical simplicity and flexibility for the flying head and its suspension. The optical path for the incident and return light beam includes a single optical fiber connected between the flying head and remotely located fixed optical components. The optical components for optically processing the polarization state of rotated polarization components of the reflected optical signal are remotely disposed from the flying head, i.e., in the drive.
The remotely mounted optical components process the rotated polarization state of the return beam signal components and present resulting differential intensity beams to a differential detector. This method relies on preserving the polarization state of the oppositely rotated signal components in the return beam through the entire optical path, including the single optical fiber, to the optical processing elements. This system provides lighter, smaller flying heads which can yield improved system data density and/or storage capacity.
The advantages of the single fiber system are tempered by certain characteristics of RF modulated Fabry-Perot (FP) laser diodes combined with the birefringent nature of the polarization maintaining optical fiber used in the optical path. Birefringence is the characteristic of many optical materials in which the index of refraction (related to the velocity of light in the medium) depends on the direction of polarization of the propagating light in the material. This will cause a phase shift between orthogonally polarized waves proportional to the difference in the refraction indices times the path length divided by the wavelength.
The relatively broad spectrum of the RF modulated FP diodes produces a multi-wavelength incident light beam. Also the wavelengths present in the incident light beam fluctuate with time (mode competition). The combination of fluctuating wavelengths passing through the single birefringent fiber produces signal components in the return beam whose polarization states are affected by the birefringent characteristics of the optical fiber. That is, the polarization angle of the return signal depends on the instantaneous wavelength of the incident and reflected beams. Since the wavelength may fluctuate about a center wavelength, the polarization state of the signal fluctuates accordingly. This fluctuation appears as noise in the differential detector. The noise limits the achievable data rate at a given signal level.
Optical fibers also experience polarization mode coupling/leakage. That is, as the light beam travels along the fiber, some of the parallel polarization component can appear as a perpendicular polarization component and vice versa. This polarization leakage will also appear at a distally located polarization detector as noise.
Light launched at a given wavelength and given polarization along an optical path having birefringent characteristics causes components along the principal axes to gradually become out of phase, i.e., polarization state of the traveling signal is not preserved. Any residual birefringent characteristic of an optical fiber will reduce signal detection sensitivity.
If the wavelength of light of a given polarization changes slightly at one end of an optical fiber, the polarization exiting other end will be slightly different from the other wavelength. Whereas, with no fiber, the polarization state would be the same for every wavelength. Therefore, if a returned polarization state is read by a polarization detector positioned at a remote end of a birefringent optical fiber, a fluctuating signal originating from the wavelength variations will be detected.
What is needed is an improved optical path for optical flying head storage systems that is compact and that allows an increase in the number of storage disks that can be placed within a given volume as compared to the prior art. The improved optical head should preferably provide a high numerical aperture, a reduced head mass, a very high resonance frequency tracking servo device, thus, producing a very fine track servo bandwidth, and be relatively easy to manufacture. Additionally, the flying optical head should improve upon: optical disk drive access times; data transfer rates; and access to, and use of, storage disk tracks, and provide for utilization of less expensive optical fiber material. The flying optical head also improves upon ease of manufacturability. Further, the improved optical head and optical path should provide reduced sensitivity to polarization mode coupling, and reduced sensitivity to laser diode mode competition.